This invention pertains to the crosslinking of water-borne resins and more particularly to a low temperature process which utilizes polycarbodiimides.
Almost every man-made item has a surface coating which serves a decorative and often additionally a protective role. One type of grouping of surface coatings is a division into systems which must be baked at high temperatures for proper curing and those which cure at ambient temperature or with low-temperature baking. These different coating types are utilized on different substrates. For example, the high-bake coatings are employed on items such as automobile fenders which are cured at high temperature. The low-bake coatings would be employed typically for wooden window frames in a home. Traditionally, all of these coating systems were solvent-based; that is, the coating components were dissolved or suspended in an organic material.
Several factors in recent years have contributed to a shift from solvent-based coatings to water-based coatings. These factors include the easier clean-up of water-borne systems, the increasing cost of solvents, and the increasing awareness of the potential harm by organic solvents to the environment. This shift has created intense efforts aimed at reformulating coatings from solvent-based to water-based systems.
For high-bake systems which are cured or crosslinked utilizing melamine or methlyolamide chemistry, the reformulation is rather straight-forward. However, in the conversion of the low-bake solvent systems to water-based formulations, many difficulties have been encountered. A description of the chemistries of the curing reactions of the low-bake solvent systems is needed for understanding of the difficulties in formulating a crosslinkable water-based low-bake coating system.
There are three representative classes of chemistries available for the curing of low-bake solvent-based systems. For systems termed moisture-cure urethanes, polymeric or oligomeric materials are prepared containing residual isocyanate functionality. A coating is prepared from these materials and applied to the particular substrate chosen. Humidity from the air diffuses into the coating, reacting with the isocyanate to release carbon dioxide resulting in the free amine which reacts rapidly with the remaining isocyanate providing the crosslinking mechanism. Conversion of this type of system to a water-based system would be difficult due to high reactivity of the isocyanate with water. A second type of curing system for low-bake solvent systems is the alkene system. In this system a polymeric or oligomeric material is formed by ionic reactions in such a manner and with such materials so as to yield residual double bond functionality. Upon formulation with a catalyst and upon exposure to atmospheric oxygen, the residual double bonds cured by free radical reactions. Most water-based polymers are prepared by free radical reactions in which the polymer preparation stage would tend to cause premature reaction to the alkene double bonds. Some systems have been designed to attempt to utilize the alkene curing reaction for crosslinking, but these systems are difficult to drive to high levels of functionality and thus high levels of crosslinking without attendant problems with reaction rate control and degree of reaction are not achieved. The third representative class of chemistries available for curing low-bake solvent-based coatings involves silicone reactions. Typically, polymeric or oligomeric materials are prepared containing alkoxysilane or acyloxysilane functionality. Upon application of the solvent solution of such materials to the substrate and exposure to the atmosphere, silanes rapidly react with atmospheric moisture to cause hydrolysis to the silanols. These silanols condense to silyloxy crosslinks. This type of system has been attempted in water-based coatings, but the presence of water along with the water-reactive silanes usually causes premature reaction.
From this description the difficulties of developing a water-based low-bake crosslinking system are apparent. Nevertheless, the need for such a system has remained strong. Water-borne low-bake noncrosslinkable coatings are currently employed, for example in interior wall paints. However, such systems are deficient in key qualities and exhibit undersirable properties, such as, high tack, low solvent resistance, and low abrasion resistance.
It is therefore an object of this invention to provide a facile system for crosslinking water-borne resins at low, that is, ambient temperatures.
It is another object of this invention to provide a low-temperature system which affords crosslinked water-borne resins suitable for coating various substrates.
It is a further object of this invention to provide low-temperature crosslinked water-borne resin coatings which exhibit low tack, high solvent resistance and high abrasion resistance.
Other objects will become apparent to those skilled in the art upon a reading of the specification.